Electrophilic Aromatic Coupling of Hexapyrrolylbenzenes. A Mechanistic Analysis

Abstract

Oxidative transformation of hexapyrrolylbenzenes into azacoronenes using bromine electrophiles as alternative coupling agents is shown to occur according to a distinct mechanism, different from those proposed for typical high-potential oxidants. It is shown that consecutive cyclizations do not involve brominated intermediates and can be rationalized by assuming a relayed arenium cation attack followed by deprotonation and conjugate elimination. The final cyclization is incompatible with this mechanism and is found to involve double-electrophilic bromination followed by thermal elimination of dibromine. These findings provide insight into the reactivity of sterically congested aromatic systems and may help in designing new methods of C–C bond formation.

Beyond the Scholl reaction – one-step planarization and edge chlorination of nanographenes by mechanochemistry

“… 9 It is also possible to perform planarization using halogenation reagents. 10 The synthesis of such graphenes is mostly performed in solution. However, caused by strong pi–pi interactions nanographenes above a certain size become increasingly insoluble.…”

Recent Advances in Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds

“…Molecular propellers C2.2a – c , chiral analogues of the snowflake-shaped C2.1 , were obtained from sterically hindered hexapyrrolylbenzenes (HPBs), which were treated with bromine electrophiles, N- bromosuccinimide (NBS), and dibromine as oxidative coupling agents. , Ferric chloride, generally an effective oxidant in the syntheses of HPHAC derivatives, applied to the sterically congested precursors of C2.2a – c , produced only mixtures of ill-defined, possibly polymeric products. Subsequent screening of various halogen electrophiles and reaction conditions showed the superior performance of NBS in lactic acid, producing C2.2a – c in 85–88% yields with excellent chemo- and stereoselectivities.…”

Novel Synthetic Approach to Heteroatom Doped Polycyclic Aromatic Hydrocarbons: Optimizing the Bottom-Up Approach to Atomically Precise Doped Nanographenes

“…An exhaustive analysis of the Br 2 /NBS promoted cyclization mechanism of substituted hexapyrrolyl benzene derivative was performed by the group of Stępieʼn. The cyclodehydrogenation of hexapyrrolyl benzene derivatives 70 can be promoted by N-bromo succinimide or Br 2 to provide chiral propeller-shaped azacoronenes 71 ( Scheme 19 ) [ 68 ].…”